In a semiconductor fab, during a photo exposure process step, a photo exposure module in a scanner focuses electromagnetic energy on a focus spot that focuses on a semiconductor wafer surface. The scanned wafer surface is made to be smooth and planar for precise positioning at the focal plane, or focus, of the scanner. The wafer surface is polished by CMP, chemical mechanical planarization, to make a smooth, planar polished surface on the wafer.
Following CMP, on the planarized wafer surface, the surface topography includes a range of taller topography features and a range of shorter topography features, of micrometer step height changes. When the taller topography features (or shorter topography features) are present in sufficient numbers to dominate the surface topography, they defocus the focus spot. The height amount that defocuses the focus spot is referred to as, a deviation amount, also referred to as a focus deviation, and also referred to as, a focus spot deviation (FSD). The photo exposure module has a wafer stage set point that will need adjustment according to the FSD to adjust the focus spot into focus on the wafer surface.
A process for determining the focus spot deviation will now be described. The wafer topography of a batch of wafers is mapped, and the mapped information is analyzed to calculate the FSD. The FSD is a relative height measure by which the focus spot would be defocused due to the height variations of the wafer topography. Then, the FSD sets a wafer stage set point for the photo exposure module. The photo exposure module is set to the wafer stage set point, which adjusts the height of the focus spot to correctly focus on successive wafers of the wafer batch. Thus, a new wafer stage set point is determined by the measure of the FSD. The batch of wafers are processed by the photo exposure module that has been adjusted to the new set point. However, the batch of wafers may have a wafer with an actual topography that would cause defocus of the focus spot, even after the photo exposure module has been adjusted to a new wafer stage set point by the amount of the FSD. Such a wafer in the batch is referred to as a defocus wafer. Thus, a defocus wafer in the batch has a topography that would need to be processed according to a wafer stage set point that is different from the permissible value of the calculated FSD that determines the wafer stage set point. Processing a defocus wafer similarly as the other wafers in the same wafer batch, would reduce the yield of the wafer batch. Thus, a need exists for an invention to identify a defocus wafer and prevent it from being processed with a defocused focus spot. Further a need exists for an invention to dynamically readjust the wafer stage set point with a corrected focus of the focus spot for processing a defocus wafer.
Prior to the invention, the wafer topography was measured by a highly accurate measuring apparatus called a level sensor apparatus, LS. The LS measured topography peak height variations per unit of surface area. According to an algorithm, an FSD was calculated using the LS measurements. The calculated results were recorded in a real time monitor (RTM) process control chart for the FSD. Prior to the invention, multiple process control charts were required. Since wafer fabrication involves a stack of successive planarized layers, the thicker the layer stacks, the bigger the value of FSD becomes. Thus, each of the successive planarized layers needed a separate RTM process control chart for focus spot deviation. Further, no RTM process chart could set a sole criteria to predict or to catch a defocus wafer.
In a photo exposure module, its wafer stage set point is determined according to the following procedure. First, level sensor apparatus measurements are taken of the topography of the wafers in a first wafer batch. Then, by applying statistics, a statistical measurement of the topography is obtained. That statistical measurement is analyzed to calculate a focus spot deviation. The process control chart is updated with the focus spot deviation. Then, the photo exposure module is adjusted to a wafer stage set point, as determined by the focus spot deviation. Then the batch of wafers are processed by the photo exposure module that has been adjusted to the new wafer stage set point.
Before the next succeeding batch of wafers are processed by the photo exposure module, new level sensor apparatus measurements are taken of their topography, to obtain a new focus spot deviation for updating the process control chart. In this manner, as the wafer topography changes from batch to batch due to process variations from batch to batch, the photo exposure module is adjusted to a new wafer stage set point. Thus, the photo exposure module adjusts to the changes in wafer topography from batch to batch. One drawback of the existing system, is that one or more wafers in a processing batch are defocus wafers, which are wafers that have greater focus spot deviations than can be compensated by the new wafer stage set point. It would be advantageous to identify the defocus wafers before they become processed by the photo exposure module. Then the photo exposure module can be readjusted with a corrected wafer stage set point that focuses the focus spot on the defocus wafer. By readjusting the wafer stage set point to process a defocus wafer, the production yield of the processed batch is desirably increased.